Purpose: Rescuing people from out-of-hospital Sudden Cardiac Arrest (SCA) depends crucially upon early defibrillation. Many Public Access Defibrillation (PAD) schemes depend upon the placement of defibrillators in fixed locations and their success may be limited by the fact that over 70% of SCAs occur in patients' own homes. A PAD scheme has recently been deployed in North West Belfast (NWB, population ~150,000) that primarily involves mobile responders.  Using data from the early phase of this project, our purpose is to explore the impact of volunteer availability upon response time distributions for reaching victims of SCA.

Methods: A PAD roster was created using a pool of 400 pre-trained lay volunteers and police officers to cover NWB for 52 weeks per year.  SCA incidence was modelled as a Poisson process, for all 26 Census Tracts in NWB, based upon data from the World Health Organization (Belfast) MONICA Project.  To minimize the potential response time distribution, lay volunteers were assigned to 7 overlapping zones, based upon where they worked or lived. A matrix of potential response times to SCA events in each of the 7 zones, based on road networks, congestion levels and average speeds was created. Actual response times for the volunteers from the first 55 verified dispatches were modelled using a log logistic distribution, as were 11,544 previous ambulance service call-out responses. In Monte Carlo simulation, we varied the proportion of call-outs that a volunteer could respond to and the number of potential volunteers available.

Results: So far, for 15% of all SCAs, a volunteer responds sooner than an emergency ambulance.  This proportion could improve by a further 19% if the number of rostered volunteers doubled; by 18%, if they were available to respond to twice as many events as they currently do; or by 32%, if their availability to respond tripled.

Conclusion: In this community, the lowering of response times by on average 56 seconds could potentially save 1 life, but ongoing commitment of volunteers to respond is crucial.